Switching power supplies, as an example of power converters, are often used in electronic devices, electronic systems, and the like. Switching power converters normally include a switch circuit comprised of: upper-arm switching elements and lower-arm switching elements; and a transformer. The switching power converters are operative to alternately turn on a selected one of the upper-arm switching elements and a selected one of the lower-arm switching elements, thus converting direct-current (DC) power input from a DC power source into different power, such as alternating-current (AC) power. The switching power converters are also operative to step up or down the AC power input to the transformer by a turns ratio of the transformer while the step-up or step-down AC power is electrically isolated from the input AC power.
In these switching power supplies, there is a technology to prevent the series-connected upper- and lower-arm switching elements from being simultaneously turned on, thus prohibiting a high-level short-circuit current from flowing through the series-connected upper- and lower-arm switching elements. This technology is designed to turn off one of the high- and low-side switching elements, delay turn-on of the other thereof by a dead time from the turn-off timing, and turn on the other thereof after lapse of the dead time.
In contrast, introducing the dead time may have an influence on the power conversion efficiency of these switching power supplies. For this reason, it is important how the dead time should be determined in view of both the reliability and power conversion efficiency of the switching power supplies.
In view of this matter, there is a technology to determine a dead time between series-connected upper- and lower-arm switching elements, which is disclosed in Japanese Patent Publication No. 3706852.
A DC-DC converter as a switching power supply disclosed in the Patent Publication is further comprised of a resonant circuit in addition to the switch circuit set forth above. The resonant circuit performs resonating operations between an inductance connected in series to the primary winding of the transformer and each of the capacitance component of a selected one of the upper-arm switching elements and that of a corresponding lower-arm switching element connected in series.
The DC-DC converter measures a value of one of: an input current thereto, which is a primary current flowing through the primary winding of the transformer; an output current output therefrom; and a current having a magnitude proportional to the magnitude of the output current. Then, the DC-DC converter determines whether a measured value of the selected current becomes equal to or lower than a constant threshold current value, and increases a dead time between one of a selected one of the upper-arm switching elements and a corresponding lower-arm switching element connected in series thereto when the measured value becomes equal to or lower than the constant threshold current value. The increase in a dead time between a selected one of the upper-arm switching elements and a corresponding lower-arm switching element connected in series thereto permits the dead time to match with a period for which the resonant operations have been completed, or to be greater than the period. This aims to reduce turn-on loss of the DC-DC converter.